The invention relates to a process for electrolytically dyeing an anodic oxide layer on aluminum or aluminum alloys by means of direct or alternating current in an electrolyte containing at least one metal salt.
To electrolytically dye aluminium or aluminium alloys, the latter are first anodically oxidized to form an oxide layer and then subjected, while the pores in the oxide layer are unsealed, to an electrolysis in an electrolyte solution which contains at least one metal salt, for example a nickel salt, in which process, as a result of the electrolytic deposition of metal or metal oxide in the pores of the deposited oxide layer, the latter is superficially dyed.
The dyeing method described is routinely applied and in principle presents no problems provided a markedly uniform dyeing is not required.
Frequently, however, the dyed coatings produced exhibit differences in shade, in particular in the case of the irregularly shaped work pieces which have projecting or recessed areas. A further disadvantage is that the uniformity and the dyeing rate decrease if a prolonged time elapses between the anodic treatment of the metal and the dyeing or if the metal is rinsed too long after anodic treatment has taken place.
To eliminate these disadvantages, a proposal has already been made several times that an anodic direct current treatment of the already anodized metal in the dyeing bath be carried out before the oxide layer is dyed in electrolytes containing metal salts. In this connection, direct current is to be understood to mean virtually similar currents, for example those produced by full-wave rectification of a single-phase alternating current or by rectification of a three-phase alternating current.
The dyeing process used in practice proceeds essentially in a manner such that the aluminum or the aluminum alloy is first anodically oxidized to form an oxide layer and the oxide layer so formed is rinsed off with water. While the pores in said oxide layer are still unsealed, the anodically oxidized metal is immersed together with a counterelectrode in an electrolyte solution. Before the electrolytic superficial dyeing takes place in this solution, a direct current is passed through the solution, the anodically oxidized metal being anodically connected in the solution. Thereafter the metal is subjected to an electrolysis, usually with alternating current, in the same electrolyte, in which process the superficial dyeing takes place.
The invention is based on the object of achieving a better uniformity of the dyeing and an increased dyeing rate in electrolytically dyeing anodic oxide layers on aluminum or aluminum alloys.